There's nothing wrong with "Happy Birthday" other than being, like you say, maybe a bit impersonal. However EL&U can't help you be more personal -- you're in the best position for that.
–
tenfourNov 2 '11 at 1:46

2

Perhaps find birthday greetings in 10 different languages, and include them all.
–
GEdgarNov 2 '11 at 13:22

What you actually say, though, should be based off your relationship with each person. If you are not very close, "Happy birthday" may be the most appropriate. If you are very close, you can perhaps play off some inside joke or personal tie to express the sentiment. "Happy birthday" is the standard well-wish, but you can vary it in a great many ways to say the same thing.

Remember, the reason you are saying this is to wish your friend well on their day of celebration, so it should be something well suited to them.

I would have upvoted twice if I could! Today's my best friend's birthday, I prepared a fine wine for him and congratulated him like this: "hope you get better as the years go by, just like the wine; and our friendship gets better and better, just like the wine." He was not prepared for that and really impressed by my wishes. Thanks again!
–
Terry LiNov 27 '11 at 5:25

It depends on your relationship with the person. I often say "Hiffty Biffty" to my spouse and I have a friend I always just say "Grats" to; congratulating them on surviving another year.

However, there are several, more accepted, standards:

Happy Birthday

Many Happy Returns

Birthday Wishes

You can also avoid "birthday" completely by just wishing them a great day or a great year to come.

You have to ask yourself - What kind of message would you like to receive on your birthday? Personally, I would rather replace an empty "happy birthday" with a sincere message of any kind (an invitation to go out for a beer, or to the cinema, perhaps?) any day!

I love to send people the links of American Cancer Society, since the celebs sing "Happy Birthday". I tailor it to the person whose birthday it is. I personally love Maroon 5, and they have a good one on there.